1. Field of the Invention
The invention relates to cellular mobile radio system networks and, more particularly, the routing of messages for roaming mobile subscribers within such networks.
2. History of the Prior Art
The public telecommunication systems can be divided generally into two components; the public switched telephone network (PSTN) and the mobile subscriber network. The latter category consists of very many mobile telephone subscribers ("Mobiles") who remain in communication with one another and with telephones connected to the PSTN by virtue of radio links within a plurality of adjacent cells. Each cell is illuminated by the radio transmissions from a particular base station which covers that cell and which is in turn coupled to a mobile switching center ("MSC") which controls the radio linkages between many different base stations and their associated cells covering a given geographic area. Each cellular exchange or cellular system is made up of one or more MSCs and its associated base stations serving the mobiles within a particular geographic area. Mobile telephone systems are connected to the PSTN to enable mobile subscribers and land base subscribers to freely communicate with one another and are interconnected with one another in different geographic areas to comprise a network.
Each mobile subscriber is associated with a particular home MSC or home location register which maintains a record of all relevant information associated with that mobile including the type of telephone service and options it has selected to receive. For example, a home MSC will store a mobile subscriber's mobile identification number ("MIN") along with its electronic serial number identifying the particular mobile instrument; the categories of service for which it is contracted, e.g., whether it has call forwarding or call waiting or whether it may originate long distance calls to certain specific areas or not; and other subscriber specific information. When the mobile subscriber is operating within its own geographic area and calls to and from the mobile are being handled through the mobile's home MSC all of this information is readily available and used by the MSC in handling telephone traffic with the mobile. Another item of information stored in the home MSC in the mobile's record is the last known geographic location of the mobile. This information enables the home MSC to locate the mobile whenever it receives a call directed to the telephone number in the home MSC assigned to the mobile.
Each mobile within the network maintains contact with the radio communication system by means of a process known as registration. In the process of registration, each mobile periodically receives a signal from a base station including data which identifies the particular system with which the mobile is in radio contact, referred to as a system identification number ("SID") in North American cellular systems. The mobile in turn responds to the periodic signals from the system base station contained within an overhead message train, including the SID signal, by sending to the system its MIN and its electronic serial number for registration with it as a user. There are basically three ways in which a mobile may register: autonomous registration (in response to the mobile entering a new geographic area), periodic registration (in response to the passage of time since the last registration), and registration while originating a call from the mobile (in response to an unregistered mobile seeking to originate a call).
When a mobile travels from the geographic area covered by its home MSC, it is said to be "roaming." In such a situation, it is desirable to be able to still provide telephone service to the mobile both in response to the mobile originating a call to be connected either to another mobile in the network or to a subscriber on the PSTN or to a call being directed from another subscriber to the mobile. Thus, it is necessary to have a way in which the mobile can be tracked to any exchange within which it may be located in the network, known as a visited exchange, outside of its home exchange. This is done by a process of maintaining records within each exchange as to the identity of all roaming mobiles currently operating within that exchange as well as the current location of all mobiles for whom that exchange is the home exchange. These data are regularly communicated between the various MSCs comprising the cellular mobile system network. This information enables the providing of telephone service to a roaming mobile by assuring the visited exchange that the mobile is authorized to be making and receiving calls in the visited exchange and that it is financially responsible for those calls. This information also provides the home exchange with data on where the mobile is currently located so that calls may be delivered from outside of the visited exchange to the mobile within that exchange.
Prior art systems provide arrangements whereby each exchange stores enough of the MIN to identify the home exchange for each mobile within the entire network which might seek to register as a roaming visitor within that exchange. Each visited exchange will, upon registration by a roaming mobile within that exchange, consult information stored within its memory, known as a digit analysis table, to determine the home exchange of the mobile in order to be able to contact that home exchange to authenticate the mobile and to request data relating to the service parameters of the mobile. Thus, each MSC within the entire network of MSCs must maintain a detailed record related to all mobiles within the network enabling that MSC to locate the home exchange of every mobile operating within the network should that mobile register as a visitor within that exchange. While such a system enables the handling of calls and the providing of service to roaming mobile subscribers, it requires the frequent updating of mobile subscriber information within a large number of MSCs as that information changes. For example, in large metropolitan areas, such as Los Angeles, in which there are a large number of different MSCs serving as home exchanges to subscribers, there are frequent revisions and changes to the service areas of each MSC because of growth in the number of subscribers and the need to reassign certain subscribers to new MSCs as the efficient capacity of particular MSCs is reached. Thus, each time the subscription for a particular mobile is moved from one MSC to another, that changes its home MSC and requires that same information to be sent to every other MSC in the network as to the new home MSC of the mobile. It is a huge undertaking to regularly update and change the memory contents of every MSC in the network every time it is necessary to update the digit analysis table of that MSC in order to render it capable of handling registrations from a mobile the home exchange of which has been changed.
The system of the present invention provides an additional node within the mobile exchange network to facilitate the handling of call registration data within the network. The provision of subscriber interrogation points within a cellular mobile telecommunications exchange network in accordance with the system of the present invention greatly simplifies the maintenance of subscriber information within the network.